86 



the first plate, 3 loopfuls for the second, and ^ c.c. 

 (measure with a graduated pipette) for the third. Place one 

 of the tubes of milk treated by each process with the plate 

 cultures, in the incubator, and leave the other tubes with a 

 tube of the fresh milk at the room temperature. 



At the next exercise note carefully the condition of the 

 milk in each of the various tubes, and also the number of 

 colonies on the agar plates. 



Keep the tubes of milk for further examination at the 

 following exercise, after which they may be rejected. 



EXERCISE LVI. 



THE QUANTITATIVE BACTERIOLOGICAL EXAMINATION OF 



WATER. 



§ 152. Explanatory note. This is to determine the num- 

 ber of bacteria in water. In preparing media for this pur- 

 pose the directions given in the Journal of American Public 

 Health Association for Jan., 1898, p. 60, should be followed. 

 The conditions of temperature and of media which favor 

 growth differ for different species. Many water bacteria will 

 not grow at the incubator temperature while others which 

 may be in it grow very slowly at the room temperature. To 

 determine numbers it is better to grow the bacteria in gelatin 

 plates at the temperature of the room. (In an actual ex- 

 amination a much larger number of plate cultures should be 

 made than can be managed here. ) 



§ 153. General directions. Make from the properly col- 

 lected water 4 gelatin plates using a definite quantity of wa- 

 ter for each. It may be safe to begin with to inoculate these 

 tubes with o.i, 0.25, 0.50, and i.oo c.c. respectively. 



To determine if there are gas producing bacteria, and the 

 approximatenumberof these if any, inoculate 10 fermentation 

 tubes with i c. c. each and 5 with^ c. c. each. (In place of 



